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// SPDX-FileCopyrightText: The futures-stream-ext authors
// SPDX-License-Identifier: MPL-2.0
use std::{
num::NonZeroUsize,
pin::Pin,
task::{Context, Poll, ready},
time::Duration,
};
use futures_core::stream::Stream;
use pin_project_lite::pin_project;
use crate::IntervalEdge;
/// Callbacks for throttling a stream
pub trait Throttler<T>: Stream<Item = ()> {
/// A new item has been received from the input stream.
///
/// After invocation of this method `throttle_ready` will be called
/// with `Some` when the current interval has elapsed. The current
/// item that will be yielded may still change until `throttle_ready`
/// is called.
///
/// The `cx` argument is only provided for consistency and can safely
/// be ignored in most cases. The throttler will be polled immediately
/// after this method returns.
fn throttle_pending(self: Pin<&mut Self>, cx: &mut Context<'_>);
/// The current interval has elapsed.
///
/// Provides the pending item of the throttled input stream that is ready
/// or `None` if no item has been received from the input stream during
/// the last interval.
fn throttle_ready(self: Pin<&mut Self>, cx: &mut Context<'_>, next_item: Option<&T>);
}
/// Internal state.
#[derive(Debug, Clone, Copy)]
enum State {
Streaming,
Finishing,
Finished,
}
pin_project! {
/// Throttled stream
#[derive(Debug)]
#[must_use = "streams do nothing unless polled or .awaited"]
pub struct Throttled<S: Stream, T: Throttler<<S as Stream>::Item>> {
#[pin]
stream: S,
#[pin]
throttler: T,
poll_next_max_ready_count: NonZeroUsize,
state: State,
pending: Option<S::Item>,
}
}
impl<S, T> Throttled<S, T>
where
S: Stream,
T: Throttler<<S as Stream>::Item>,
{
pub const fn new(stream: S, throttler: T, poll_next_max_ready_count: NonZeroUsize) -> Self {
Self {
stream,
throttler,
poll_next_max_ready_count,
state: State::Streaming,
pending: None,
}
}
}
impl<S, T> Stream for Throttled<S, T>
where
S: Stream,
T: Throttler<<S as Stream>::Item>,
{
type Item = S::Item;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let mut this = self.project();
if matches!(this.state, State::Streaming) {
// Poll the inner stream while it yields items. We want to receive
// the most recent item that is ready.
let mut ready_count = 0;
loop {
match this.stream.as_mut().poll_next(cx) {
Poll::Ready(Some(item)) => {
if this.pending.is_none() {
this.throttler.as_mut().throttle_pending(cx);
}
*this.pending = Some(item);
debug_assert!(ready_count < this.poll_next_max_ready_count.get());
ready_count += 1;
if ready_count >= this.poll_next_max_ready_count.get() {
// Stop polling the inner stream to prevent endless loops
// for streams that are always ready.
// Wake ourselves up to ensure that polling the stream continues after
// after polling the throttler.
cx.waker().wake_by_ref();
break;
}
}
Poll::Ready(None) => {
*this.state = State::Finishing;
break;
}
Poll::Pending => {
break;
}
}
}
}
// Poll the throttler.
match this.state {
State::Streaming => {
ready!(this.throttler.as_mut().poll_next(cx));
let next_item = this.pending.take();
this.throttler
.as_mut()
.throttle_ready(cx, next_item.as_ref());
if next_item.is_some() {
Poll::Ready(next_item)
} else {
Poll::Pending
}
}
State::Finishing => {
if this.pending.is_some() {
ready!(this.throttler.as_mut().poll_next(cx));
let last_item = this.pending.take();
// Wake ourselves up for the final state transition from `Finishing`
// to `Finished` that becomes ready immediately.
cx.waker().wake_by_ref();
Poll::Ready(last_item)
} else {
// The final state transition.
*this.state = State::Finished;
Poll::Ready(None)
}
}
State::Finished => panic!("stream polled after completion"),
}
}
}
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct ThrottleIntervalConfig {
/// Throttling period
///
/// The minimum interval between subsequent items that limits the
/// maximum frequency of the output stream.
pub period: Duration,
/// Interval edge gate
///
/// Controls whether the pending item of the stream is yielded
/// immediately or after the interval has elapsed.
pub edge: IntervalEdge,
}